How well will Earthquake Early Warning work? (Reader Poll)

Update: It now appears that the earthquake early warning system did not go off prior to shaking and that the video below has been altered

Wouldn’t it be great to be warned? Shaking from a scenario M=7.8 quake (dashed black line) on the southern San Andreas Fault reaches Los Angeles about 80 seconds after the rupture nucleates in the Salton Sea. Source: Geoffrey Ely, Temblor.

In the M=7.1 Puebla, Mexico earthquake on September 19, which caused significant damage in Mexico City, the city’s earthquake early warning sirens went off approximately 12 seconds before the strongest ground shaking (See video below – sirens go off at the 37 second mark). While the onset of shaking appears to begin only a few seconds after the sirens, that should have been enough for people to take cover underneath tables or desks. Coincidently, this earthquake occurred on the 32nd anniversary of a devastating M=8.0 earthquake, the quake that prompted the creation of the Earthquake Early Warning system. This is the second earthquake early warning that the capital has received in the last two weeks. The September 7 M=8.1 Chiapas earthquake, which caused little to no damage in Mexico City was triggered a 60 second warning.

Although this is the type of earthquake early warning that people desire, Mexico City is unique: The capital city is located on a geological “water bed,” and so gets shaken even by earthquakes 400 km (250 mi) away. In all likelihood, earthquake early warning elsewhere will contend with uncertainties and false alarms, or at least nuisance alerts. Just as important, in most instances that matter, we will only get a few seconds of warning, just enough time to ‘drop, cover, and hold on.’

This Temblor figure highlights how much of Mexico City is susceptible to extreme soil amplification during earthquakes.

Stirring up the hornet’s nest

At the annual meeting of the Southern California Earthquake Center last week, USGS seismologist Sarah Minson took a deeper look at what we can expect in the U.S., focusing on ideal performance under perfect conditions, so that its theoretical limits can be understood. Earthquake Early Warning lies at the unexplored interface of seismology and sociology, and so to make the warnings beneficial, both its capabilities and weaknesses must be probed. Her presentation triggered a vigorous discussion by the proponents and skeptics of this important technology.

Three types of Warning Systems

Around the world, several countries, including the United States, are either in the process of developing, or have already deployed, earthquake early warning systems. While each system has its own unique traits, they all rely on the ability to detect different types of seismic waves, and alert people before strong shaking arrives. The fastest seismic waves are P waves, which are generally undetectable to humans, but can be picked up by high quality seismometers. Next are S waves, which cause strong ground shaking and can result in significant damage to man-made structures. Because P waves travel roughly 60% faster than S waves, if the P wave can be reliably detected, in theory one should be able to alert people that damaging S waves will soon arrive. The farther away from the quake you are, the longer the warning time, but also the weaker the shaking. So, there is a ‘sweet spot’ that optimizes the timeliness and value of the warnings.

In a nutshell, there are three types of earthquake early warning systems. The simplest of these is a stand-alone P-wave detector that alerts users if a P wave is detected (Wu et al., 2013). This approach does not depend on telemetry or fast cloud computing, and can be used anywhere in the world. But rarely, it can be ‘spoofed’ by vibrations when there is no earthquake, and it can give no indication of how strong the shaking will be. In Taiwan, where 500 Palert instruments have been in place for a decade, with warning times of 4-5 seconds for most locations that were strongly shaken by the M=6.5 quake in 2016. That’s enough time to duck for cover, and so they worked.

The second type of system involves a network of sensors that detects S waves that are at least 30 km (20 mi) away. As more seismographs detect these S waves, updated projections of how far the waves will travel and how strong they will be can be given. This is how earthquake early warning will soon be implemented in Japan’s second-generation system (‘PLUM’) (Kodera et al., 2016). The prototype has been successful in warning before subduction zone earthquakes, since they are offshore and so provide long warning times. The last type of system, ShakeAlert, is what the USGS, UC Berkeley, Caltech, and University of Washington and University of Oregon are jointly developing (Strauss and Allen, 2016). ShakeAlert, which will have a limited rollout next year, is designed to not only determine the location and magnitude of the earthquake, but will also alert people how strong the shaking will be like at their location.

The difficulty of warning only for strong shaking

But here’s the nut of Dr. Minson’s message: In order to give sufficient warning, the shaking threshold for the alert has to be set very, very low. So, while earthquake early warning could very well save lives and reduce injuries, getting useful alerts comes at a steep cost—and that’s quite apart from money. Most alerts you receive will not be followed by strong shaking, and some shaking will be imperceptible. In these instances, the alert could be considered a nuisance. While the shaking threshold required to trigger an alert could be raised, doing so would increase the “blind zone,” the area in which no warning could be given. So, if you only want to be warned when there will be strong ground shaking (say, because scramming your factory is costly or disruptive), you will almost never be forewarned.

We are tracking clicks, nothing will change on the screen when you vote. We will place the results here at the end of the week.

65% of people prefer a longer warning

Nuisance alerts or teachable moments?

What all of this means is that for earthquake early warning to work, we have to accept the idea that several times a year, we’ll get an alert when shaking is very light. Some may see these as an inconvenience, and others could lose confidence in the system.

But there is a flip side to this: Perhaps these warnings provide a natural opportunity to prepare for what we should do in the event of an earthquake. In a sense, these might serve as unscheduled and unannounced fire alarms. That could help all of us train for what we’ll need to do with several seconds of warning so we protect ourselves in the event of strong shaking. Don’t run out of the house or office, and don’t call Mom. Instead, dive under a table or desk. This preparedness training could be the real pearl of Earthquake Early Warning: In the end, it’s not the technology but the muscle memory that will count.

It’s unfortunate that the timing of yesterday’s earthquake, 32 years to the date from 1985, coincided with drills and other reminders for Mexico City residents, so when the sirens went off people may have thought it was part of the day’s program.

The Leaf

I think they are teachable moments indeed. WE NEED IT FOR SURE

John Vidale

While much of this article is on the mark, this statement – “we have to accept the idea that several times a year, we’ll get an alert [for dangerous shaking] when shaking is very light” seems wrong to me. Earthquakes that people in a given spot feel as light shaking do not generally occur several times a year. I’ve been on the West Coast for 35 years, and outside of aftershocks and swarms, felt maybe 3 or 4 quakes. And earthquakes that people feel as light shaking, nearby M3s, should be too small to generally make false alarms of severe shaking in an EEW system.

Ross Stein

This is a good point, Polat, and hard to assess. In the linked video, none of the drivers or pedestrians seem to be responding to the beginning of the siren. Perhaps they thought it was a drill.

Ross Stein

Thank you (John is a USC professor and Director of the Southern California Earthquake Center). You inserted ‘[for dangerous shaking],’ but we did not mean that dangerous shaking would be felt several times a year. Rather, depending on where you live, this might be the frequency of light shaking from moderate-small local shocks or moderate-larger remote shocks. California examples might include San Bernardino (SoCal) or Mendocino (NorCal). Beyond that, we defer to the experts of this technology; they should feel free to reply.

John Vidale

I’d agree that if people for fun, earthquake awareness, or just curiosity set an EEW threshold to capture earthquakes that anyone around SoCal, for example, could feel, they would receive many alarms. But if they ask to alerted only to dangerous earthquakes in their location, the alert level should be much less than several times a year.

Robin Luethe

Cognitively it may take a second or two to respond and start to act for any warning. This could be put to use. The earliest warning advises us to get ready to act, the next warnings could tell us how severe the shaking might be. If the second screen said ‘minor’ we could relax just a bit.

Ross Stein

In rather informal tests we’ve conducted, most people can dive under a nearby table or desk in less than 4 sec. So, we think EQ Early Warning will work best if one always assumes the shaking will be strong. That’s why the prototype ShakeAlert screen makes no sense to us: No one should look at a map of expanding circles, or read the shaking level legend, or count it down. That’s nuts: Just act!

It will be fascinating and important to know how many lives were saved in Mexico City because of their warning system, and how many lives were lost closer to the epicenter because of the large blind zone.

John Vidale

I don’t know the details, but a 50-km deep earthquake with a West Coast style ShakeAlert algorithm would not have a blind zone – weak P leads stronger S by enough, 5-10s, for interpreting P to yield a decent warning of the S wave.

Also, the demonstration ShakeAlert app, as I understand, was designed to make clear what was being resolved about the earthquake to show its capabilities. It was generally intended to have real warnings recommend actions more than show a map with mesmerizing expanding concentric circles.

Robin Luethe

LOL – at my age I am not going to dive under a table unless I think I really need to – while I am fairly athletic I also have injured knees!

Alex Hutko

Whenever the study by Dr Minson is discussed it should be emphasized that it focuses on shallow crustal earthquakes, i.e. California. Washington and Oregon could also have these type earthquakes, but could also have 50km deep earthquakes (more warning time) as John pointed out or subduction zone earthquakes (up to a minute or more warning time). This point is understood by us seismologists, but can easily be missed by non-experts.

Ross Stein

My understanding is that even for remote earthquakes, it will take 30-60 sec to know if they will be very large (because great quakes rupture for a longer duration but at speeds similar to small ones), and so a warning of ‘moderate or greater shaking’ will still be brief. But for these remote events, long (30 s) warning times are possible if for ‘very light or greater’ shaking.

John Vidale

This raises the difference between predicting the shaking from the rupture that has already happened compared to probability of shaking from rupture that has not yet happened but is likely to come. The best case (in some sense) for Seattle is a rupture that has broken from Cape Mendocino north to Salem, and might have a 50% chance to run all the way to the Canadian border. So that’s a 50+% chance of an M9 passing Seattle, by Chris Goldfinger’s statistics. That’s a 50+% chance, with more than a minute of warning, of strong shaking.

Hardin Rich

One of the weakness I envision in this is that if used for “fun’ or in “curiosity” this interest will soon dissipate into complacency.

Ross Stein

John, I hope you’re right, but this would be a very special case. In the 1980’s, we at the USGS bet the farm on Parkfield on the San Andreas being a very special case, but it didn’t work out. Today, Meier et al. was published in Science (http://science.sciencemag.org/content/357/6357/1277). They find that large and small subduction events grow at the same rate until they begin to peter out, making early detection of their size, and so shaking levels, very difficult.

Pablo Ampuero

On a brighter side, our study also shows that, as long as the earthquake moment rate is still growing, we can expect its seismic moment will grow at least twice bigger. We’re assessing if this idea is practical despite the random fluctuations relative to the average earthquake behavior.

Tom Heaton Caltech

The ShakeAlert team is planning to provide as much warning and as specific a shaking prediction on a second-by second basis as is allowed using data available at the time of the alert. If a prediction of severe shaking is required to initiate an action, then this will only occur when the rupture is nearby and the warning time will necessarily be short. Currently, we are planning to track the rupture in real time (see the FinDer algorithm of Boese and others); since we are not yet able to predict future rupture of an ongoing event. Although it’s likely that many strongly shaken areas will get 10’s of seconds of warning in a long rupture, it will not be possible to predict that strong shaking is likely at that site until the rupture has propagated close to the site. Interestingly, the same situation plays out in Hurricane alerting (but over a much longer time scale); a region knows days ahead that an earthquake is headed its way, but the final prediction of landfall is not known until it’s about to happen.

Anticipating the performance of ShakeAlert can be done by simulating thousands of hypothetical events. However, the reality of the next several decades will depend on just several significant earthquakes that we are likely to be surprised by. Sometimes we will do well and it’s likely that we’ll have cases where the public is disappointed with our performance. We have all seen this happen in the ups and downs of weather alerting. Nevertheless, we can anticipate that our efforts will provide new information that was never before available in earthquakes.

Ross Stein

Thank you, Tom. This is very helpful. But you wrote, “If a prediction of severe shaking is required to initiate an action, then this will only occur when the rupture is nearby and the warning time will necessarily be short.” My understanding of Sarah Minson’s SCEC talk is that even under ideal circumstances, there will be no warning possible for severe or greater (≥0.5 g) shaking, and only a matter of a few seconds for moderate or greater (≥0.2 g) shaking.

John Vidale

Nisqually in 2001 is a perfect example in which the earthquake can have a good characterization from the P wave well before the S wave arrives – deep earthquakes allow warning. The PNW coastal M9 is another case in which the waves can be damaging far from their source, even if one is not probabilistically forecasting the rupture progression. Perhaps >0.5g shaking would not be forecast in the Seattle basin, but certainly strong and long-duration long-period motions can be confidently forecast.

Didn’t Mexico City just demonstrate that warning is possible for deep quakes? What in Sarah’s dataset represented the deep quakes that are the most frequent hazard in the PNW, and did those have the sort of station coverage we’re planning?

Ross Stein

I cannot speak for Sarah or her coauthors. But there is conflicting information coming in from Mexico City for the Puebla M=7.1 shock warning. In the linked video above, the siren sounds a few sec before shaking begins, and 12 s before peaking shaking. But oddly, neither pedestrians nor cars seem to respond to the siren for about 10 s. Our translator’s post, on the other hand, indicates that people downtown felt strong shaking about 12 s before the siren; when the sirens began, everyone was already on the street. Could the soundtrack in the video we posted have been shifted? Yesterday, Doug Wiens and I invited a leading Mexican seismologist to speak at the Dec 12 Seismology-Tectonophysics luncheon at the Fall AGU meeting on the warning system’s performance.

John Vidale

Interesting. Sarah and Men’s approaches are the correct one – take real records or good simulations of earthquakes rather than more idealized models to get the actual performance. It also can bypass the idealized concept of the arrival time of the S wave, which may or may not correspond with the onset of strong shaking or peak shaking.

Ross Stein

I believe its now clear there was no warning in Mexico City for the M=7.1. Here’s what Eduardo Miranda, Prof. of Civil & Environmental Engineering at Stanford, wrote me about the M=7.1 warning: “The Mexican early warning system is primarily designed to catch large events along the subduction trench along the coast. Although there was one SAS station very near the epicenter of the 9/19/2017 M=7.1 event, the SAS algorithm requires confirmation from several stations, which, combined with a much shorter distance, meant that the public warning was probably issued after the arrival of S waves into Mexico City. People were already feeling a strong earthquake by the time the sirens went off.” Here’s what Germán Acosta, our translator wrote me: “I asked some friends at work, and all agree first was the shaking, then, short after that, the sirens started. The sirens sound aloud enough that there’s no mistake about that.”

John Vidale

Hmm. One would have hoped the people operating the system would have clarified the performance more quickly and decisively. In any case, for an earthquake 100 km away and 50 km deep, it would not be hard with sufficient instrumentation and standard algorithms to generate warnings before the strong shaking, albeit with a more comprehensive system than currently deployed in Mexico.